In these days, a modulation system used in a wireless communication system employs two baseband signals, which are at right angles to each other, at the time of transmission and reception in order to enhance frequency usage efficiency. The two signals are called an I signal and a Q signal. The I signal and the Q signal are modulated at the time of transmission and are demodulated at the time of reception. At this time, an quadrature mixer is required, which correctly demodulates 90-degree phase difference between the I signal and the Q signal, on a receiving side.
FIG. 9 illustrates an quadrature mixer described in Patent document 1. FIG. 2A is a diagram illustrating an operational waveform of a Local Oscillator (LO) signal thereof. The mixer employs clock signals (CLKI, CLKQ, CLKIB, CLKQB) each having a duty ratio of 25%, phases of which are shifted by 90 degrees each other, instead of LO (LO_I, LO_Q) signals with a duty ratio of 50%, phases of which are shifted by 90 degrees each other. As a result, a circuit (transistor) which converts a voltage signal in a received Radio Frequency (RF) band into a current is shared on the I side and the Q side. That is because it is necessary to avoid the problem that an error occurs in amplitude of both of the modulated I signal and the modulated Q signal if each conversion gain is fluctuated, when a voltage-current conversion circuit is separately arranged at both of the I side and the Q side.
As shown in FIG. 2B, even when the phase difference between LO_I and LO_Q deviates from 90 degrees, phase relations of four-phase clocks generated thereby is characterized in correctly differing by 90 degrees each other. Therefore, an IQ signals being correctly at right angle each other can be demodulated.
Patent document 2 describes a configuration of a wireless circuit with a mode which adjusts amplitude errors of the I signal and the Q signal, as shown in FIG. 21. In the configuration, in an idle slot, an output signal of an orthogonal modulator directly enters an orthogonal demodulator as a test signal. A technology is disclosed, in which an adjustment value is updated so that amplitude of the I signal and that of the Q signal are equalized.